#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform float u_time;

// 放大倍数、再缩小
vec2 brickTile(vec2 _st, float _zoom){
    _st *= _zoom;
    _st.x += step(1., mod(_st.y, 2.0)) * 0.5;
    return fract(_st);
}

float box(vec2 _st, vec2 _size){
    _size = vec2(0.5)-_size*0.5;
    vec2 uv = smoothstep(_size, _size+vec2(1e-4), _st);
    uv *= smoothstep(_size, _size+vec2(1e-4), vec2(1.0)-_st);
    return uv.x*uv.y;
}

void main(void){
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(0.0);


    // http://www.jaharrison.me.uk/Brickwork/Sizes.html

    st /= vec2(2.15, 0.65)/1.5;

    st = brickTile(st, 10.0);

    color = vec3(box(st, vec2(0.95)));

    //    color = vec3(st, 0.0);

    gl_FragColor = vec4(color, 1.0);
}
